Write your first Flutter app on the web

I've just finished the following tutorial: https://flutter.dev/docs/get-started/codelab-web


The tutorial doesn't explained in details how Flutter application are build. It gives however a first idea of the general structure of the app:
- Widget are immutable
- Widget can be statefull or stateless
- When a widget is statefull, the state is stored in an associated private class (private class start with an underscore)
- The `Navigator` widget manage which scree/page/route should be display. This is done by changing (adding, removing using `setState` function) element to a stack of widget. The widget on the top of the stack is the one which is displayed
- callback functions are used to define the behaviour of the application. For example `onPressed` event call the function...

As expected the application works on Chrome and on Android and uses the same code.
I think the difficulty will be know how to search the flutter API and build from it the intended UI. The API is doing a lot for you as it contains a lot of predefine UX.

I'm going to continue with other smaller tutorial to get more experience and understanding how Flutter works before continuing with #15

Looking at: [Write your first Flutter app, part 1](https://flutter.dev/docs/get-started/codelab)